It has been awhile but when I need 2 media players to run at once, in addition to WACUP, I will fire up my original Winamp5.666 install. I have stopped using WACUP's preview build because there are too many fixes to it in the beta version and I exclusively use the beta. I know not if it can run more than one beta instance. Anyways, as I said, I fired up the 5.666 and the attached image popped up.
Just curious and I did dismiss it.